The present invention relates to an image forming apparatus provided with the functions of a photocopier, printer, facsimile and multi-functions thereof.
One of the techniques known in the conventional art is an image forming apparatus where a toner image is formed on the photoreceptor drum as an image carrier using electrophotographic process and the toner image is transferred onto a sheet of paper, and then the image is fixed by a fixing section.
In this image forming apparatus, when an image forming operation has started, the sheets stored in a sheet accommodation section are sequentially fed to a sheet conveyance path by a sheet feeding unit.
This sheet conveyance path is provided with a registration roller for nipping and conveying sheets and a loop forming roller arranged on the upstream side of this registration roller to nip and convey the sheets.
Each of the aforementioned registration roller and loop forming roller is made up of a pair of rollers composed of a drive roller and a driven roller.
The sheet fed out from a sheet feed section by the sheet feeding unit is conveyed by plural conveyance rollers including the loop forming roller arranged on the upstream side of the registration roller. The leading edge of the sheet is made to hit against the registration roller whose rotation has been suspended. After that, a loop is formed on the sheet by the rotation of the loop forming roller. This allows all area of the leading edge of the sheet to hit against the registration roller, whereby sheet skew is corrected.
After this correction of sheet skew, the registration roller starts to rotate synchronously with formation of an image on the photoreceptor drum, and the sheet is fed again. In this sheet re-feed operation, the sheet is conveyed while the sheet is kept looped by the rotating loop forming roller and is nipped by the registration roller.
In the aforementioned sheet re-feed process, width-wise bias of the sheet is corrected.
With respect to the aforementioned skew correction and bias correction, a proposal has been made of a sheet conveyance apparatus (see, e.g., Japanese Unexamined Patent Application Publication No. 2008-265974). According to this proposal, the leading edge of the sheet being conveyed is made to hit against the registration roller, and then the sheet is fed out to the next process downstream from the conveyance path, with the start of rotation of the registration roller. A guide member having a loop shape and arranged to extend in the direction perpendicular to the sheet conveyance direction is rotatably provided close to the upstream side of the registration roller.
In the Japanese Unexamined Patent Application Publication No. 2008-265974, the leading edge of the sheet having been conveyed by the conveyance rollers including the loop forming roller arranged upstream of the registration roller is made to hit against the registration roller whose rotation has been suspended. Further, a loop is formed on the sheet by the rotation of the loop forming roller, whereby skew of the sheet is corrected.
After the sheet skew correction, synchronously with image formation on the photoreceptor drum, the registration roller and loop forming roller are rotated at approximately the same speeds and the sheet is re-fed with the loop kept formed.
In the aforementioned sheet re-feeding step, bias of the sheet is corrected by width-wise traveling of the registration roller. In the aforementioned step of registration roller traveling, the aforementioned guide member is rotated in such a direction as to encourage displacement of the sheet that is displaced as a result of traveling of the registration roller, whereby distortion of sheets is minimized.
When the leading edge of the skewed sheet has been hit against the registration roller to correct skew, a difference occurs in the size of the loop on both ends across the width perpendicular to the sheet conveyance direction. To be more specific, there is a difference in the shape of the loop. The difference in the shape of the loop is greater as the skew is increased.
In the following description, the loop having a different shape is also called the uneven loop.
FIGS. 2a and 2b are schematic diagrams illustrating the sheet skew and skew correction. FIG. 2a shows that the sheet P is skewed and is conveyed by the loop forming roller 22D. FIG. 2b shows that, after the leading edge of the sheet P has been hit against the registration roller 23 whose rotation is suspended, a loop is formed on the sheet by the rotation of the loop forming roller 22D, and the sheet skew is corrected.
As shown in FIG. 2b, when the sheet P is skewed and the leading edge of the sheet P is made to hit against the registration roller 23, and then a loop is formed to correct skew, the loop will have different sizes in the cross-wise direction perpendicular to the sheet conveyance direction, i.e., on the downside of the FIG. 2b (also referred to as “on the near side” in the following description) and on the upside of the FIG. 2b (also referred to as “on the far side” in the following description). FIG. 2b shows an example wherein the loop L1 on the near side is greater than the loop L2 on the far side.
After the leading edge of the sheet has been hit against the registration roller, and skew has been corrected, the registration roller starts rotation. Then the sheet is flipped out by the bias force in the sheet conveyance direction, which is generated by the loop due to the toughness of the sheet P, and the sheet is nipped by the registration roller. The sheet is further fed out downstream while the loop is maintained by the rotating loop forming roller.
The aforementioned bias force differs according to the shape of a loop. The bias force is greater as the loop is smaller. Thus, the difference in the bias force is increased as the shape (size) of the loop is increased.
Accordingly, when the rotation of the registration roller has started and the sheet is fed by the registration roller and loop forming roller, the amount of conveyance on the smaller loop side (FIG. 2b, loop L2) is greater than that on the greater loop side (FIG. 2b, loop L1) under the influence of the difference in the bias force of the uneven loop. Thus, the sheet is conveyed with the leading edge kept in the shape of a fan.
FIG. 3 is a schematic diagram illustrating that a sheet is conveyed with the leading edge kept in a fan shape. A difference of distance d1 occurs on the right and left of the leading edge of the sheet P, and the fan shape shown by oblique lines (hatching) is created.
If the sheet is conveyed with such a fan shape, the positional accuracy of images on the front and rear surfaces will especially be deteriorated, in the case of duplex printing where the front surface is first printed and then the sheet P is reversed to perform printing on the rear surface.
FIG. 4 is a drawing showing the positions of images when a fan shape such as one shown in FIG. 3 has been produced on both surfaces in the duplex printing mode. In the drawing, the solid line indicates the front surface image position, and the broken line shows the rear surface image position.
As described above, in the Japanese Unexamined Patent Application Publication No. 2008-265974, a rotatable guide member is used to correct skew and bias of the sheet, whereby distortion of the sheet is suppressed. However, action is not taken to remove a difference in the amount of conveyance due to a difference in loop shape, i.e., to suppress formation of the fan shape. This method cannot easily improve the positional accuracy in printing, in particular, that in the duplex printing.
To minimize such image misalignment, it is preferred to release the sheet from the nipping of the loop forming roller 22D as quickly as possible so that the sheet can be conveyed only by the registration roller 23, without being adversely affected by the uneven loop.
The position of the leading edge with respect to the registration roller is different in conformity to the type of a sheet. The aforementioned type of the sheet denotes the differences in paper thickness, and differences in whether or not a toner image has been formed on the first surface of the sheet.
(1) In skew correction, when the leading edge of the sheet has been hit against the registration roller, the position of the leading edge with respect to the registration roller is different due to the difference in paper thickness.
FIG. 5 is a diagram illustrating the contrast in the respective positions of the leading edges when the leading edges of the thin sheet and thick sheet have been hit against the registration roller 23. It should be noted that FIG. 5 does not represent the actual diameter of the registration roller or actual thicknesses of the thin sheet and thick sheet and the values in FIG. 5 are exaggerated. In the drawing, a solid line indicates the leading edge of a thin sheet, while the broken line shows the leading edge of a thick sheet.
As shown in FIG. 5, when the leading edge of the sheet has been hit against the registration roller 23, the leading edge of the thin sheet comes closer to the nip section N formed by the drive roller 23a and driven roller 23b, than that of the thick paper. This produces a difference in distance d21.
Thus, after the rotation of the registration roller 23 has started, the sheet is conveyed downstream with the loop kept formed, and if the sheet is released from the nipping of the loop forming roller 22D after the lapse of a constant time, independently of the thickness of paper, the amount of conveyance before this release from the nipping is greater in the case of a thin sheet than that in the case of a thick sheet. To be more specific, the distance from the center of the registration roller nip to the leading edge of the sheet is greater in the case of a thin sheet than that in the case of a thick sheet. Thus, the fan-shaped portion is increased in size when a thin sheet is conveyed. To put it another way, the influence of the uneven loop is increased.
To avoid this, if a step is taken to reduce the time before the sheet is released from the nipping of the loop forming roller 22D, the amount of conveyance will be insufficient when the thick sheet is conveyed, and nipping at the registration roller 23 will be insufficient. Thus, failure of secure conveyance may be caused by nipping failure.
(2) In skew correction, when the leading edge of the sheet has been hit against the registration roller, the position of the leading edge with respect to the registration roller differs according to whether or not a toner image is formed on the first surface of the sheet.
As described above, each of the registration roller 23 and loop forming roller 22D is composed of a pair of rollers consisting of a drive roller and driven roller. The drive roller is placed at the position in contact with the surface opposite to the surface where an image is formed, while the driven roller is located at the position in contact with the surface where an image is formed.
In the following description, the term “opposite surface” indicates the surface of the sheet in contact with the drive roller, unless otherwise specified.
FIG. 6 is a diagram showing the positional relationship between the drive roller 23a and the driven roller 23b of the registration roller 23.
In the duplex printing mode where images are formed on both sides of a sheet, after an image has been formed on the first surface of the sheet, the sheet is reversed to change the positions of the first and second surfaces. Then the sheet is again conveyed to the loop forming roller. Similarly to the case of forming an image on the first surface, sheet skew is corrected.
Depending on whether or not an image is formed on the surface (opposite surface) of the sheet in contact with the drive roller 23a, namely, depending on the presence or absence of a toner image on the first surface, the friction coefficient on this opposite surface varies. Friction coefficient is smaller when there is a toner image than the case when there is no toner image.
Accordingly, after the rotation of the registration roller 23 has started, the sheet is conveyed downstream with the loop kept formed. If independently of presence or absence of a toner image on the opposite surface, the sheet is released from the nipping of the loop forming roller 22D after the lapse of a constant period of time, the amount of conveyance before the sheet is released from the nipping will be greater when there is no toner image on the opposite surface than when there is a toner image. The broken line of FIG. 6 indicates the position of the leading edge when there is no toner image on the opposite surface. The solid line denotes the position of the leading edge when there is a toner image on the opposite surface. As described above, depending on the presence or absence of a toner image, there is a difference d22 in the distance from the nip center of the registration roller 23 to the leading edge of the sheet.
Thus, when there is no toner image, the size of the fan-shaped portion is increased. To put it another way, there is an increase in the influence of the uneven loop.
To avoid this, if a step is taken to reduce the time before the sheet is released from the nipping of the loop forming roller 22D, the amount of conveyance will be insufficient when there is a toner image on the sheet, and nipping at the registration roller 23 will be insufficient. Thus, failure of secure conveyance may be caused by nipping failure.
In view of the problems described above, it is an object of the present invention to provide an image forming apparatus by which, in the step of conveying a sheet after skew thereof has been corrected by hitting the leading edge of the sheet against the registration roller and forming a loop, stable conveyance is ensured by preventing conveyance failure from occurring due to influence of the difference in the shape across the width of the loop formed at the time of skew correction.